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Estimate yields in your fields

A few years back, when I was a new agronomist in Avoca, one of my customers asked me in mid-August to do yield estimates in his cornfields. This sounded like great fun, but I still had to ask him why. His response — “So I don’t have to do it” — was classic.

Nov 01, 2010

Estimate yields in your fields

By CLARKE McGRATH

A few years back, when I was a new agronomist in Avoca, one of my customers asked me in mid-August to do yield estimates in his cornfields. This sounded like great fun, but I still had to ask him why. His response — “So I don’t have to do it” — was classic.

But further into the conversation he explained how a good estimate of his yields a month or more ahead of harvest benefited his farming operation.

He said many top growers like to make plans for transport, drying, storage, handling and marketing of the crop prior to harvest to get “ahead of the game.” His rationale made a lot of sense to me, and I have been a proponent of making quality preharvest crop yield estimates ever since.

Undoubtedly, any field estimates made this year will be interesting, to say the least, given the wide range of crop conditions.

Estimating corn yields

Corn yields can be estimated as early as the “milk,” or R3, stage of kernel development, roughly three weeks after the end of pollination. You’re correct: It is hot, humid and itchy out in the cornfields that time of year, but the discomfort may be worth the benefits to your operation.

There are several methods for estimating corn yields, but most agronomists like this formula: Multiply the number of ears by the number of kernel rows and number of kernels per row, and then divide by 90. The resulting value is the estimated yield in bushels per acre.

The 90 in this formula represents the average number of kernels in a bushel of corn (in thousands). You can substitute a smaller value, like 80 for 80,000 kernels or 85 for 85,000, if conditions during grain fill have been excellent (larger kernels, fewer per bushel). Conversely, if grain fill conditions have been stressful, a larger value like 95 or 100 can be used (smaller kernels, more per bushel).

How many samples to take is based on field size, crop uniformity and cropping practices. I typically recommend at least six stops in a relatively uniform field, and quite a few more stops in many fields.

At each stop, you will measure 1/1,000th of an acre of row and record the harvestable number of ears in that section. For every fifth ear in the sample row, record the number of complete kernel rows per ear and average number of kernels per row. It is recommended to separately sample fields with different soil types, hybrids, tillage systems, rotations, etc.

When done well, this method can be accurate to around plus or minus 10% to 15%. Some agronomists say this is an exercise in futility, but my experiences with corn yield estimates have been pretty good.

It’s important to avoid grabbing ears in the best-looking parts of the field; doing that will skew yield estimates higher than they should be.

Estimating soybean yields

After I got relatively good at corn yield estimates, I got cocky and thought I should try soybeans. More on how that went in a minute. If you want to try, it’s recommended you wait until around growth stage R6 (full seed, pods have green seeds that fill the pods on the upper part of the plant).

Here is the formula: Multiply number of plants per acre by number of pods per plant and number of seeds per pod, and then divide by quantity of seeds per pound. Take the resulting number and divide by 60 (approximate pounds of soybeans per bushel). The result is the estimated soybean yield per acre.

Good luck. Using the formula the first time, one of my customers and I estimated 176 bushels per acre. We laughed and agreed that a visual evaluation of the field told us 50 bushels per acre was more realistic. Apparently, wide variations in soybean plant populations, seeds per pod, pods per plant and seed size across fields make this formula more art than science.

After fine-tuning my soybean yield estimates, I still don’t come very close with the formula. Because it is really tough to estimate bean yields, I recommend going golfing or fishing, catching a high school football game, or doing whatever else you want to do for the rest of August. Have a safe and profitable harvest season.

McGrath is the partner program manager and Extension agronomist for Iowa State University’s Corn and Soybean Initiative.

This article published in the August, 2010 edition of WALLACES FARMER.